One practice which may be employed when drilling a borehole is to enlarge a hole with an expandable reamer. This may be done as a separate operation to enlarge an existing borehole drilled at an earlier time, or it may be done when using a bottom hole assembly which has a drill bit at its bottom end and an expandable reamer positioned at some distance above the drill bit.
When a bottom hole assembly contains both a drill bit and a reamer, the drill bit makes an initial hole, sometimes referred to as pilot hole. It is normal that a drill bit has so-called gauge pads (alternatively spelt “gage pads”) on a part of the drill bit above the cutters. These pads are positioned so that they present faces confronting the wall of the drilled hole at the same radial distance from the drill bit axis as the radial extremity of the cutters, so that these pads can press on the wall of the newly drilled pilot hole but without enlarging it because they do not have cutting surfaces. The purpose of these pads is to position the drill bit in the newly drilled hole.
The purpose of the reamer (which is higher up in the assembly) is to enlarge the diameter of the pilot hole which has already been drilled. An expandable reamer makes it possible to insert the reamer into a hole with a diameter which is less than the expanded diameter of the reamer and then expand the reamer at a desired depth so as to commence a section of borehole which is reamed out to a larger diameter than the pilot hole and possibly also larger than the hole above the reamer. Both the drill bit and the reamer may consist of supporting structure with hard-surfaced cutters attached to the supporting structure.
Another frequent use of an expandable reamer is to enlarge the diameter of an existing borehole. For this the expandable reamer may be used in an assembly without a drill bit because the purpose is to enlarge but not extend the existing hole.
There is more than one type of reaming tool. Some reamers are constructed to be eccentric, relative to the drill string to which they are attached and the borehole which they are enlarging. Other reamers are constructed to remain concentric with the drill string and the borehole. These different types of reamers tend to be used in different circumstances. There are many instances where concentric reamers are the appropriate choice.
An expandable reaming tool is normally made with plurality of radially expandable support elements for cutters arranged around the axis of the tool. Often the tool has three such cutter support elements which extend axially along the tool and are arranged at 120° intervals azimuthally around the tool axis. A mechanism is provided for expanding these support elements radially outwardly from the axis and this mechanism typically uses hydraulic pressure to force the cutter support elements outwardly.
This tool construction has commonly been used for concentric reamers. In some constructions, each of the individual cutter support structures arranged around the tool axis is an assembly of parts attached together so as to move bodily as one piece, in which case the assembly is often referred to as a “block” (one part of this assembly may be a shaped block) although the term “arm” has also been used for such a block. The individual support structures (i.e. individual blocks) may be moved outwards in unison by one drive mechanism acting on them all, or may be moved outwards by drive mechanism(s) which does not constrain them to move in unison.
U.S. Pat. No. 7,975,783 shows a different construction in which each of the cutter support elements arranged around the axis of the tool has two parts which are pivotally connected, and expansion causes outward movement of the pivot.
When the reamer, with its cutter support elements retracted, is in position at the required depth, the drill string is rotated for a period of time at approximately constant depth while the reamer is expanded. There is not normally any communication from the reamer to the surface to confirm that its cutter support elements have fully expanded, but an indication of expansion of the reamer may come from observing the torque on the drill string. Once it is believed that the reamer is fully expanded, the drill string and bottom hole assembly are advanced axially so that the expanded reamer travels axially, cutting as it goes and enlarging the diameter of a length of the borehole.
It will be appreciated that whilst the reamer is being expanded its cutters are required to cut radially outwardly from the axis of the borehole but once the reamer has been expanded and the drill string and bottom hole assembly are being advanced axially, the reamer is cutting in an axial direction.
It is normal practice that the cutter support elements of an expandable reamer are provided with stabilising pads which have faces positioned to confront the wall of the borehole and press on the newly-reamed borehole wall. The purpose of these stabilising pads is to position the axially advancing reamer in the hole in a manner analogous to the gauge pads on a drill bit keeping the bit positioned in a pilot hole. However, there is an inherent conflict of functions. The pads are intended to press on the wall while the reamer is being advanced axially, but they also press against the wall of the borehole whilst the reamer is being expanded and in consequence the pads oppose the expansion of the reamer's cutter support elements.
A customary approach has been to make the stabilising pads slightly under-gauge, that is to say they are positioned at a radial distance from the central axis of the reamer which is slightly less (perhaps by only 1 mm) than the radial distance from that axis to the extremity of the cutters, the outermost of which define the gauge radius of the reamer. Consequently, the outermost cutters project radially slightly beyond the stabilising pads and it is found that the reamer can cut radially outwards as it is being expanded. The process by which a reamer with under-gauge stabilising pads cuts radially outwardly as it is being expanded is not clear. Even though under-gauge, the pads would be expected to oppose expansion and it may be necessary to provide some axial movement of the reamer in the borehole, perhaps by moving the drill string up and down slightly, or advancing it slowly. When the fully expanded reamer is advanced axially, these stabilising pads will engage the wall of the borehole sufficiently to keep the reamer in position, even though the pads are under gauge. If a reamer does not have these stabilising pads, its position in the borehole is much less stable.
A number of prior documents make proposals for stabilization of reamers, for instance by placing a stabilizer in the drill string close to the reamer as in U.S. Pat. Nos. 8,205,689 and 8,297,381. U.S. Pat. No. 8,550,188 describes a reamer in which the cutter support elements are distributed asymmetrically around the tool axis as a way to enhance stability. In contrast, U.S. Pat. No. 7,954,564 seeks to enhance stability by decreasing imbalance force during longitudinal drilling. This is done partly through the arrangement of cutters on the support elements and partly through elongating the stabilising pads to 30 to 45% of the total length of a cutter support element.